1. Field of the Invention
The present invention concerns an industrial aqueous alkaline electrolyte open secondary storage battery, i-e. one having a high capacity in the range 10 Ah to 200 Ah and "open" in the sense that it operates at a low pressure (less than 1 bar relative). Storage batteries of this kind are in particular of the nickel-cadmium (Ni--Cd) or nickel-metal hydride (Ni--MH) type.
2. Description of the Prior Art
An open industrial storage battery includes an electrode assembly comprising a plurality of electrode pairs consisting of a positive electrode, a negative electrode and a separator that is only slightly permeable to gases disposed between the positive and negative electrodes, together with an alkaline electrolyte in which the assembly is immersed and the level of which is above the top edge of the electrodes. On overcharge, aqueous electrolyte storage batteries generate oxygen at the positive electrode and hydrogen at the negative electrode. An open storage battery operates at a relative pressure (pressure difference relative to atmospheric pressure) less than 1 bar and the gases generated on overcharge escape, consuming water from the electrolyte. The storage battery therefore requires regular maintenance, i.e. water must be added periodically. The maintenance frequency depends on the operating conditions of the storage battery in the application concerned, in particular on the charged capacity.
To avoid topping up the electrolyte level after periods of operation sealed industrial type storage batteries have been derived from those previously described (U.S. Pat. No. 5,576,116). A sealed industrial (high-capacity) storage battery includes an electrode assembly comprising a plurality of electrode pairs consisting of a positive electrode, a negative electrode and a gas-permeable separator disposed between the negative and positive electrodes, a limited quantity of alkaline electrolyte and an oxygen recombination device. The oxygen formed at the positive electrode increases the pressure inside the storage battery which depends on the overcharge conditions employed. Permanent conditions are established thereafter in which the oxygen produced at the positive electrode is reduced, or recombined, at the negative electrode. A sealed industrial storage battery has a safety valve operating at a relative pressure higher than 1 bar. Although a sealed storage battery can solve the technical problem associated with maintenance, its energy per unit mass and per unit volume are lower than those of an open storage battery. On the one hand the sealed storage battery has a precharge and an excess of negative capacity designed to prevent the release of hydrogen at the end of charging. On the other hand such storage batteries contain a small quantity of electrolyte and consequently the yield of the active material is lower than in open storage batteries.
It has therefore appeared desirable to work towards reducing the maintenance of open storage batteries. U.S. Pat. No. 5,128,217 proposes self-limitation of the charge of an Ni--Cd storage battery based on the sharp increase in the voltage at the end of charging. This open industrial storage battery operates at a relative pressure less than 1 bar and contains excess electrolyte.
The aim of the present invention is to propose an open industrial storage battery requiring no maintenance with energy per unit volume and per unit mass higher than those of a sealed industrial storage battery.